1. Field of the Invention
The present invention is directed to a magnetic stimulation apparatus of the type having at least one stimulation coil that has terminals connected to the output of a current-generating unit.
2. Description of the Prior Art
Magnetic stimulation apparatuses serve for magnetic stimulation of nerve fibers and muscle tissue in the field of medical diagnostics and therapy. Compared to electrical stimulation with stimulation current, the advantage of magnetic pulse stimulation lies in the lower pain of the stimulation, since no higher current densities in the region of the pain receptors of the skin occur given magnetic pulse stimulation. A further advantage of magnetic stimulation lies in the higher penetration capability, as a result whereof the excitation of tissue lying deeper is also possible, particularly nerve fibers that lie deeper.
The book by R. F. Schmidt (editor), xe2x80x9cNeuro-und Sinnesphysiologiexe2x80x9d, Springer, second, amended edition 1995, Chapters 2 and 3, contains an exact description of neuro-physiological occurrences. The nerve system, for example, coordinates the activities of the various organs and reactions of the body to the environment. This mainly occurs due to modifications of the potential of nerve cells. All cells have a quiescent potential. At the quiescent potential, all membrane currents of a cell are in equilibrium. When the membrane potential is depolarized by an additional membrane current that, for example, proceeds into the cell due to an external influence, then this is accompanied by a change in potential, referred to as an action potential. The aforementioned, depolarizing membrane current is also called stimulus. The trigger potential for an action potential is called threshold. The equilibrium of the membrane currents changes at threshold. Additional membrane currents that depolarize the membrane occur for a short time. This condition is also called excitation. An action accompanies an action potential. Thus, for example, each spasm of a muscle fiber is accompanied by an action potential in the muscle fiber, and each reaction of a sensory cell to a sensory stimulation is accompanied by action potentials.
European Application 0 182 160 discloses an apparatus for generating electromagnetic pulses with a semicircular shape and a frequency of 100 Hz that, in particular, serves for promoting the micro-circulation of the blood in the region of the hair roots and the skin, for example to prevent hair loss. To that end, a diode rectifier bridge in Graetz circuitry that feeds a pulse-generating coil is connected to an A.C. voltage transformer.
German OS 36 07 235 discloses an apparatus for generating unipolar air ions and electromagnetic pulse fields for reducing the human reaction time while simultaneously increasing the attention readiness. For generating the electromagnetic pulse fields, a frequency generator that generates a frequency in the range from 8 Hz through 10 Hz and having a following out-coupling amplifier and a coil generating the pulse field is connected to a voltage source.
German OS 41 32 428 discloses a magneto-therapy apparatus for magneto-therapeutic treatment. For generating a pulse setting magnetic field with a pulse repetition rate between 0.25 Hz and 2 Hz, an unstable multi-vibrator is connected to a battery, this multi-vibrator feeding two cylindrical coils filled with iron. The apparatus is fashioned as a pocket apparatus.
U.S. Pat. No. 5,743,844 discloses an apparatus for therapy with pulsating electromagnetic fields for promoting healing of bone and body tissue, particularly in an embodiment as a battery-powered apparatus that can be worn on the body. To that end, a coil generating the magnetic field is supplied from two voltage sources of different voltage height via a specific circuit that contains two field effect transistors and two capacitors as critical elements. The aforementioned circuit thereby has a fixed pulse-to-pause time relationship.
The devices disclosed in the above-cited documents are all designed such that the magnetic pulsed or, alternating fields they generate act on the human body below the threshold for triggering action potentials. The effects in the human body that can thus actually be achieved are partly very diffuse and scientifically controversial. Magnetic stimulation devices that intentionally trigger action potentials, particularly in more deeply disposed neural-muscular tissue, however, constitute an entirely different category of devices. Not only are the use and therapeutic effect of these devices different but they operate at multiply higher electrical powers to be provided, this being reflected in correspondingly high current and voltage values. The devices disclosed in the above-cited documents are not suitable for this purpose due to their overall low-voltage and micro-current-oriented design.
A magnetic stimulation apparatus for triggering action potentials, in more deeply disposed, neuro-muscular tissue as well, is described in the article by M. Schmid, T. Weyh and B. -U. Meyer, xe2x80x9cEntwicklung, Optimiereung und Erprobung neuer Gerxc3xa4te fxc3xcr die magnetomotorische Stimulation von Nervenfasernxe2x80x9d, Biomedizinische Technik, 38 (1993), pages 317 through 324. It comprises a stimulation coil to which current pulses generated in resonant fashion are supplied. The current-generating circuitry required for generating the current pulses comprises a controllable power pack part as well as a high-voltage capacitor that forms a parallel resonant circuit together with the stimulation coil, i.e. it operates as a resonant circuit. The high-voltage capacitor is charged by the controllable power pack part and thereby accumulates the pulse energy required for the output of a current pulse.
The resonant frequency of the parallel resonant circuit formed by the stimulation coil and the high-voltage capacitor is defined by the selection of the capacitance of the high-voltage capacitor and by the inductivity of the stimulation coil and lies in the range from 1 through 3 kHz. When the capacitance of the high voltage capacitor is varied, then the resonant frequency of the parallel resonant circuit and, thus, the rate of the current rise in the stimulation coil can be modified. The stimulus intensity is defined by the initial voltage at the high-voltage capacitor. Only the repetition rate, which lies in the area around 10 Hz, can be set as a further parameter.
Further, German OS 196 07 704 A1 discloses an apparatus for magnetic excitation of neural-muscular tissue. The known apparatus comprises an excitation coil (stimulation coil) that forms a parallel resonant circuit together with a storage capacitor (high-voltage capacitor), i.e. likewise works as 5 resonant circuit. Given this apparatus as well, resonant frequencies can only be realized in the range from 1 through 3 kHz.
An object of the present invention is therefore to create a magnetic stimulation apparatus for triggering action potentials in more deeply disposed neural-muscular tissue as well that offers greater degrees of freedom in the selection of the current pulse shapes.
The above object is achieved in an inventive magnetic stimulation apparatus having at least one stimulation coil that has terminals connected to the output of at least one current-generating unit, whereby the current-generating unit offers non-resonantly generated current pulses for the stimulation coil.
By abandoning resonant operation, greater degrees of freedom can be achieved in the selection of the current pulse shapes. Moreover, no regulatable power pack parts having specific charging circuits are required.
Since the stimulation coilxe2x80x94by contrast to the comparable magnetic stimulation devices of the prior artxe2x80x94is not part of a parallel resonant circuit, further degrees of freedom derive due to the selection of the inductivity of the stimulation coil.
In an embodiment the current-generating unit includes at least one controllable power converter having at least one power semiconductor switch with short switching times that can be switched on and off.
As used herein the term xe2x80x9cshort switching timexe2x80x9d means switching times of approximately 1 xcexcs or less.
The power semiconductor switches that can be switched on and off and that are provided in the aforementioned embodiment must exhibit short switching times, so that transistors, particularly IGBTs or MOSFETs, are preferably currently employed therefor.